Scanning device including a partly plastic high-NA objective system

ABSTRACT

An optical scanning device for scanning an information layer ( 104 ) of an optical record carrier ( 102 ) includes a radiation source ( 110 ) for generating a radiation beam ( 108 ) and a high-NA objective system ( 118 ) for converging the radiation beam on the information layer. The objective system includes a first lens ( 116 ) and a second lens ( 117 ). The first lens includes a glass body and the second lens is made of plastic. The signs of the temperature-dependences of the spherical aberration of the first and second lens are different, thereby reducing the temperature-dependence of the spherical aberration of the objective system as a whole.

The invention relates to an optical scanning device for scanning anoptical record carrier, to an objective system for use in such ascanning device and a method for manufacturing such an objective system.

In optical recording a higher information density on the optical recordcarrier must be accompanied by a smaller radiation spot for scanning theinformation. A smaller spot can be realised by a higher numericalaperture (NA) of the objective system used for focussing a radiationbeam in the scanning device on the record carrier. An example of ahigh-NA objective system as used for instance in a so-called digitalvideo recorder (DVR), includes two plano-aspherical lenses mounted in acylindrical mount. The two lenses are made by the so-called glass-2Pprocess. The objective system can be easily assembled and allows bothtilt alignment and inter-lens distance adjustment without affecting thedecentring of the two lenses (see U.S. Pat. No. 6,487,026 (PHNL000132)and U.S. Pat. No. 6,510,011 (PHNL000269)). The first lens facing theradiation source has a size which is similar to conventional lenses madefor single-lens objective systems as used in the so-called DVD and CD-RWscanning devices. The second lens facing the record carrier is muchsmaller in size. A disadvantage of the known objective system is thedifficult manufacture of the second lens using the glass-2P process.Another disadvantage is the relatively expensive assembly of threecomponents, two lenses and a mount, to one objective system.

It is an object of the invention to reduce the manufacturing cost of thehigh-NA objective system. It is another object of the invention tomaintain the possibility for tilt alignment and easy lens assemblywithout affecting the decentring of the lenses.

The first object is achieved, when according to the invention, the firstlens of the objective system includes a glass body and the second lensis made of plastic and the signs of the temperature-dependences of thespherical aberration of the first and second lens are different. Thefirst lens can be made in a conventional way, for example by glassmoulding or by the glass-2P process. The second lens can be made byrelatively low-cost plastic injection moulding.

A problem introduced by the use of a plastic component is thetemperature stability of the objective system, the demands on whichbecome higher with increasing numerical aperture. The invention residespartly in the recognition that a deviation of the temperature of a lensfrom the design temperature introduces spherical aberration as a mainaberration or changes an already existing spherical aberration of thelens. To control the spherical aberration, the further insight is used,that the temperature-dependence of the spherical aberration of a lens(i.e. the derivative of the spherical aberration with respect to thetemperature) can be made positive or negative. In other words, when thetemperature of the objective system changes, an increase of thespherical aberration of one of the lenses of the objective system isaccompanied by a decrease of the spherical aberration of the other lens.The counteracting changes of the spherical aberration of the two lensescompensate at least partially the temperature-dependence of thespherical aberration of the objective system as a whole.

The compensation is particularly advantageous for objectives systemshaving an NA higher than 0.65, which are relatively sensitive totemperature changes. The magnitudes of the temperature-dependence of thespherical aberration of the first and second lens are preferably madesubstantially equal, such that the temperature-dependent sphericalaberration of the objective system as a whole induced by a 30 degreechange in temperature is preferably lower than 30 mλ after compensation,making the lens suitable for use in many demanding applications.

An advantage of the invention is that the temperature compensation doesnot require additional optical elements in the scanning device but canbe incorporated in the two lenses of the objective system, which twolenses are already present because of manufacturing reasons. Moreover, acompensation within the objective system is preferred over acompensation by another optical element such as a collimator lens. Sincethe temperature of the objective system is determined in part by theheating of the actuator close to the objective system, the temperatureof the collimator and the objective system need not be equal, resultingin an incorrect temperature compensation of the objective system by thecollimator. This problem does not arise when the lenses of the objectivesystem themselves are mutually compensated for temperature changes.

In a preferred embodiment the second lens is integrated with the mountfor the first lens, so that the second lens and the mount can be made inone injection moulding process. The integration of the relatively smallsecond lens and the mount eases the handling of the second lens. If themount has a cylinder shape, then the mounting of the first lens in thiscylindrical mount can be done in the same way as explained in U.S. Pat.No. 6,487,026. This method is possible when the glass body of the firstlens (glass-2P lens) has a thickness which is larger than the radius ofthis glass body (see also U.S. Pat. No. 6,510,011). As a result asignificant cost reduction is achieved, since the second lens can bemanufactured easier with the injection moulding technique and there arenow only two components which have to be assembled instead of three whenthe both lenses are glass-2P products. This achieves the second objectof the invention.

The insight that the temperature-dependence of the amount of sphericalwavefront aberration introduced by a lens is a function of themagnification of the lens can be used to adapt the magnification of thelens so as to give the temperature-dependence of its sphericalaberration the desired sign.

The invention also relates to an optical scanning device for scanning aninformation layer of an optical record carrier, the device including aradiation source for generating a radiation beam and an objective systemfor converging the radiation beam on the information layer, theobjective system including a first and a second lens, wherein, accordingto the invention, the first lens includes a glass body and the secondlens is made of plastic and the signs of the temperature-dependence ofthe spherical aberration of the first and second lens are different.

A further aspect of the invention relates to a method for manufacturingan objective system having a numerical aperture larger than 0.65 forfocussing a radiation beam having a wavelength λ and including a firstlens made of glass and a second lens made of plastic, including thesteps of designing the objective system to have a temperature-dependenceof the spherical aberration of less than 30 mλ OPDrms for a temperaturechange of 30 K by making the signs of the temperature-dependence of thespherical aberration of the first and second lens different and themagnitudes of the temperature-dependence of the first and second lenssubstantially equal, manufacturing the first and second lens accordingto the design and assembling the first and second lens to an objectivesystem.

The invention will now be described in greater detail by way of examplewith reference to the accompanying drawings in which:

FIG. 1 shows an objective system consisting of two elements;

FIG. 2 shows the wavefront aberration OPDrms of the objective systemarising due to a temperature rise of 30 K as a function of themagnification β of the second lens;

FIGS. 3, 4 and 5 show the wavefront aberration of the objective systemwhen the second lens is made of PMMA, COC and PC, respectively, as afunction of the magnification β of the second lens; and

FIG. 6 shows a device for scanning an optical record carrier includingan objective system.

FIG. 1 shows an embodiment of the objective system according to theinvention, which allows tilt alignment and easy lens assembly withoutaffecting the decentring of the lenses. The objective system comprises afirst lens which includes a substantially spherical lens body enclosingmore than a half of this spherical lens body and a second lens made ofplastic and a cylindrically shaped plastic mount. The opticallyeffective diameter of the first lens is larger than that of the secondlens. The first lens of the embodiment in the Figure is a glass-2P lens1, made of a glass body 2 and a thin aspheric layer 3 of cured lacquer.The second lens 4 is integrated in a cylindrical mount 5 of the sameplastic material to form a single constructive unit. The thickness d andthe radius r of the spherical glass body of the first lens 1 complieswith d>r. Element 6 is the objective system in the form of the assembledfirst lens, the mount and the second lens. In the Figure the radiationbeam enters the objective system from the top, and the exiting beamfocuses at a position below the objective system. The information layerof an optical record carrier may be arranged at this position forscanning this layer by the spot formed by the exiting beam.

The embodiment of the objective system of FIG. 1 has an entrance pupildiameter of 1.5 mm, a record-carrier-side numerical aperture NA=0.85, adesign wavelength of 405 nm, a free working distance (FWD) between thelens and the disc of 0.15 mm, and a transparent layer of the recordcarrier made of PC with thickness 0.1 mm.

FIG. 2 shows the behaviour of the second lens when its temperature ischanged. The Figure shows the change of the wavefront aberration of theentire objective system when the temperature of the second lens only isincreased by 30 K. The aberration is plotted as a function of themagnification β of the second lens. When changing the magnification ofthe second lens, the imaging properties of the objective system as awhole are maintained by a corresponding change in the properties of thefirst lens and in the distance between the first and second lens. Thewavefront aberration is presented as the well-known root-mean-squarevalue of the optical path difference (OPDrms). A negative value of theOPDrms in the Figure means that the Zernike coefficient for sphericalaberration, A₄₀, is negative. The three lines in the Figure show thebehaviour of the aberration for the case that the second lens is made ofpolymethyl methacrylate (PMMA), cyclic olefin copolymer (COC) andpolycarbonate (PC). The thermal properties of PMMA, COC and PC are givenin Table I. The change in spherical wavefront aberration is positive andlarge for a lens having an infinite conjugate (β=0). For a magnificationβ greater than approximately 0.3 the wavefront aberration is negative.For 0.2<β<0.8 the wavefront aberration due to a temperature rise isrelatively small. The reason for this is that the magnification is closeto the range determined by the magnification corresponding to theHuygens aplanatic condition (β˜1/n², with n refractive index of secondlens) and the magnification corresponding to the centre of curvaturecondition (β˜1/n). Since the glass-2P lens has a lower temperaturedependence than the plastic lens, the magnification of the plastic lensin the high-NA objective design should be in the range 0.2<β<0.8 toachieve a low temperature dependence of the combination of the glass-2Plens and the plastic second lens.

TABLE I Thermal properties of PMMA, COC and PC. PMMA COC PC n @ 405 nm1.5022 1.5499 1.6223 dn/dT [10⁻⁵/K] −12.5 −10.0 −10.8 Coeff. Lin.Expansion 70 70 70 [10⁻⁵/K]

The invention allows the temperature tolerance of a glass-plastichigh-NA objective to be made sufficiently wide for optical recordingpurposes, which will also become apparent from the following moredetailed examples.

An embodiment of the objective system for use in a DVR scanning deviceoperates at infinite conjugate; a first lens faces the radiation sourceof the scanning system and a second lens faces the record carrier. It isdesigned for focussing a collimated radiation beam having a wavelengthof 405 nm to a focus on an information layer of an optical recordcarrier through a transparent layer of PC (n=1.6223) with thickness 0.1mm and arranged on the information layer. The entrance pupil diameter is3.0 mm and the numerical aperture of the objective NA=0.85. The freeworking distance is FWD=0.15 mm. The objective system consists of aplano-aspheric glass-2P first lens followed by a plano-aspheric secondlens made of plastic. The glass body of the first lens is made of FK5Schott glass (n=1.4989) with a thin aspheric lacquer layer on itssurface and made of Diacryl (n=1.5987). The thickness on the opticalaxis of the first lens is fixed to 2.3 mm. Alternatively, the first lensmay be made entirely of glass, using for example glass moulding. Thedesign of the objective system is optimized for different materials andmagnifications of the second lens using a merit function based onrequirements regarding field tolerance, decentre tolerance of the twolenses, the tolerance for disc thickness variation with conjugateadjustment allowed and the temperature tolerance.

The embodiment of the objective system having a second lens of COC and amagnification β of 0.4 has the following design parameters. The firstlens has a convex surface facing the radiation source and having aradius of curvature R_(base) of 2.4 mm. The rotationally symmetricalaspherical shape of the lacquer layer on the convex surface is given bythe equation:

${s\left( \theta_{N} \right)} = {\sum\limits_{i = 1}^{8}\;{a_{2i}\theta_{n}^{2i}}}$in which

$\theta_{N} = {\theta\frac{R_{base}}{r_{A}}}$and s is the deviation of the local thickness of the lacquer layer fromits thickness at the optical axis and measured in millimeters along thenormal to the surface on a point of the convex surface, θ is the anglebetween the optical axis and a radius from the centre of curvature ofthe convex surface to said point on the convex surface and measured inradians, r_(a) is the semi-diameter of the convex surface measured inmillimeters and a_(2i) is the coefficient of the 2i^(th) power or θ. Thevalue of r_(a) is equal to half the entrance pupil diameter. Thethickness of the lacquer layer on the optical axis is 0.015 mm. Thevalues of the coefficients a₂ to a₁₆ are 0.10705588, −0.22546373,0.087850329, 0.079496556, −0.14937823, −0.21393161, −0.5056136 and1.2663043, respectively. The boundary face of the first lens oppositethe convex surface has an infinite radius of curvature. The second lensof the objective system is made of COC; it has a thickness of 1.363 mmon the optical axis and a distance of 0.100 mm to the first lens. Theconvex surface of the second lens directed towards the first lens has arotationally symmetrical aspherical shape given by the equation:

${z(r)} = {\sum\limits_{i = 1}^{8}\;{b_{2i}r^{2i}}}$in which z is the position of the surface in the direction of theoptical axis and measured in millimeters, r is the distance to theoptical axis in millimeters, and b_(2i) is the coefficient of the2i^(th) power of r. The values of the coefficients b₂ to b₁₆ are0.49982075, 0.12648361, −0.015903608, 0.27317405, −0.47765099,0.39496049, −0.0085041792 and −0.10275875, respectively. The boundaryface on the other side of the second lens has an infinite radius ofcurvature.

When analysing the temperature dependence of the optical properties ofthe objective system, the expansion of the mount made of the samematerial as the second element was taken into account. FIGS. 3, 4 and 5show the change in wavefront aberration of the objective system when thesecond lens is made of PMMA, COC and PC, respectively, as a function ofthe magnification β of the second lens element. Taking 30 mλ OPDrms asthe upper limit for the aberrations, β is preferably smaller than 0.5for all cases due to the combined effect of the temperature-inducedspherical aberration and decentre tolerance. In the case of PMMA thefield tolerance limits the magnification to β>0.26, while for COC and PCno further limitations arises.

For ease of manufacture the magnification β of the second lens elementmust not be too small, because for small β most of the optical power ofthe system is then present in the second lens, while the first lens hasonly small optical power. As a result the magnification β must be largerthan 0.3. When combined with the upper limit of the previous paragraph,a preferred range from 0.3 to 0.5 is obtained.

Furthermore, the Figures show that the best performance is obtained whenusing COC as plastic material for the second lens. In this case a gooddesign is obtained when β is in the range from 0.35 to 0.45, andpreferably substantially equal to 0.4.

Although the above embodiments show the lens in the objective systemhaving the smaller magnification to be closer to the radiation sourcethan the lens having the larger magnification, the lens having thelarger magnification may also be arranged closer to the radiation sourcethan the lens having the smaller magnification.

The objective system can be used in a scanning device of an opticalrecording system, for instance in the so-called DVR optical system.

FIG. 6 shows a device 101 for scanning an optical record carrier 102 ofthe DVR type. The record carrier comprises a transparent layer 103, onone side of which an information layer 104 is arranged. The side of theinformation layer facing away from the transparent layer is protectedfrom environmental influences by a protection layer 105. The side of thetransparent layer facing the device is called the entrance face 106. Thetransparent layer 103 acts as a substrate for the record carrier byproviding mechanical support for the information layer. Alternatively,the transparent layer may have the sole function of protecting theinformation layer, while the mechanical support is provided by a layeron the other side of the information layer, for instance by theprotection layer 105 or by a further information layer and a transparentlayer connected to the information layer 104. Information may be storedin the information layer 104 of the record carrier in the form ofoptically detectable marks arranged in substantially parallel,concentric or spiral tracks, not indicated in the Figure. The marks maybe in any optically readable form, e.g. in the form of pits, or areaswith a reflection coefficient or a direction of magnetisation differentfrom their surroundings, or a combination of these forms.

The scanning device 101 comprises a radiation source that can emit aradiation beam 108. The radiation source shown in the Figure comprises asemiconductor laser 110. A beam splitter 113 reflects the divergingradiation beam 108 on the optical path towards a collimator lens 114,which converts the diverging beam 108 into a collimated beam 115. Thecollimated beam 115 is incident on a first lens 116 and subsequently ona second lens 117 of an objective system 118. The objective system maycomprise two or more lenses and/or a grating. The objective system 118has an optical axis 119. The objective system 118 changes the beam 115to a converging beam 120, incident on the entrance face 106 of therecord carrier 102. The objective system has a spherical aberrationcorrection adapted for passage of the radiation beam through thethickness of the transparent layer 103. The converging beam 120 forms aspot 121 on the information layer 104. Radiation reflected by theinformation layer 104 forms a diverging beam, transformed into asubstantially collimated beam 123 by the objective system 118 andsubsequently into a converging beam 124 by the collimator lens 114. Thebeam splitter 113 separates the forward and reflected beams bytransmitting at least part of the converging beam 124 towards adetection system 125. The detection system captures the radiation andconverts it into electrical output signals 126. A signal processor 127converts these output signals to various other signals. One of thesignals is an information signal 128, the value of which representsinformation read from the information layer 104. The information signalis processed by an information processing unit for error correction 129.Other signals from the signal processor 127 are the focus error signaland radial error signal 130. The focus error signal represents the axialdifference in height between the spot 121 and the information layer 104.The radial error signal represents the distance in the plane of theinformation layer 104 between the spot 121 and the centre of a track inthe information layer to be followed by the spot. The focus error signaland the radial error signal are fed into a servo circuit 131, whichconverts these signals to servo control signals 132 for controlling afocus actuator and a radial actuator respectively. The actuators are notshown in the Figure. The focus actuator controls the position of theobjective system 118 in the focus direction 133, thereby controlling theactual position of the spot 121 such that it coincides substantiallywith the plane of the information layer 104. The radial actuatorcontrols the position of the objective lens 118 in a radial direction134, thereby controlling the radial position of the spot 121 such thatit coincides substantially with the centre line of a track to befollowed in the information layer 104. The tracks in the Figure run in adirection perpendicular to the plane of the Figure.

1. An optical scanning device for scanning an information layer of anoptical record carrier, the device including a radiation source forgenerating a radiation beam, a collimating lens system for forming theradiation beam into a collimated radiation beam and an objective systemfor converging the collimated radiation beam on the information layer,the objective system including a first and a second lens, characterisedin that the first lens includes a glass body and the second lens is madeof plastic and the signs of the temperature-dependence of the sphericalaberration of the first and second lens are different.
 2. The opticalscanning device according to claim 1, wherein the objective system has anumerical aperture larger than 0.65 and the magnitudes of thetemperature-dependence of the spherical aberration of the first andsecond lens are substantially equal such that the temperature-dependenceof the spherical aberration of the objective system is smaller than 30mλ OPDrms for a temperature change of 30 K.
 3. The optical scanningdevice according to claim 1, including a detection system for convertingradiation coming from the information layer to an information signal,and an information processing unit for error correction of theinformation signal.
 4. An objective system including a first and asecond lens for focussing a collimated radiation beam through atransparent layer on the optical record carrier and onto an informationlayer, characterised in that the first lens includes a glass body andthe second lens is made of plastic and the signs of thetemperature-dependences of the spherical aberration of the first andsecond lens are different.
 5. The objective system according to claim 4,wherein the objective system has a numerical aperture larger than 0.65and the magnitudes of the temperature-dependence of the sphericalaberration of the first and second lens are substantially equal suchthat the temperature-dependence of the spherical aberration of theobjective system is smaller than 30 mλ OPDrms for a temperature changeof 30 K.
 6. The objective system according to claim 4, including a mountfor the first lens, wherein the mount and the second lens are integratedinto a single piece.
 7. The objective system according to claim 4,wherein the first lens has a magnification β smaller than 0.2 and thesecond lens has a magnification β larger than 0.2.
 8. The objectivesystem according to claim 4, wherein the second lens has a magnificationβ smaller than 0.6.
 9. The objective system according to claim 4,wherein the second lens is made of cyclic olefin copolymer.
 10. Theobjective system according to claim 9, wherein the second lens has amagnification β in the range 0.35<.beta.<0.45.
 11. A method formanufacturing an objective system having a numerical aperture largerthan 0.65 for focussing a collimated radiation beam having a wavelengthλ and including a first lens made of glass and a second lens made ofplastic, including the steps of designing the objective system to have atemperature-dependence of the spherical aberration of less than 30 rmkOPDrms for a temperature change of 30 K by making the signs of thetemperature-dependence of the spherical aberration of the first andsecond lens different and the magnitudes of the temperature-dependenceof the first and second lens substantially equal, manufacturing thefirst and second lens according to the design and assembling the firstand second lens to an objective system.
 12. The system of claim 4,wherein the plastic lens includes an aspherical layer of cured lacqueron the glass lens.
 13. The system of claim 4, wherein both areaspherical.
 14. The system of claim 13, wherein both lens areplano-aspherical.
 15. The system of claim 4, wherein both lens havepositive optical power.
 16. A scanning device comprising: at last oneradiation source for providing a radiation beam; at least one means forcollimating the radiation beam from the radiation source and radiationreflected from a record carrier; at least one objective system asrecited in claim 4 and disposed to cause the collimated radiation beamto converge on the record carrier, the objective system inducing aspherical aberration adapted to correct for passage of the radiationbeam through a transparent layer of the record carrier; and at least onemeans for detecting and processing reflected radiation from the recordcarrier.
 17. The device of claim 16, further comprising at least onemeans for feeding back an error signal to the objective system; and atleast one means for adjusting the lenses of the objective systemresponsive to the error signal to achieve better position or focus ofthe radiation beam on the record carrier.
 18. The device of claim 16,wherein both lens are of the objective system have positive power andare plano-aspherical.